Fischer-Tropsch synthesis operations produce hydrocarbons having a wide range of molecular weights. In general, Fischer-Tropsch hydrocarbons as recovered from the Fischer-Tropsch reactor may be divided up into three categories based roughly upon their boiling ranges. However, it should be understood that there is considerable overlap between the individual boiling ranges of each of the three categories. The heaviest fraction includes hydrocarbons boiling above about 680° F. and generally is referred to as Fischer-Tropsch wax. Fischer-Tropsch wax is usually solid at ambient temperature. An intermediate fraction boiling between ambient temperature and about 700° F. is referred to as condensate. The condensate fraction is a liquid at ambient temperature and contains hydrocarbons boiling within the range of naphtha and diesel. The lightest fraction is normally gaseous at ambient temperature and contains light hydrocarbons, i.e., hydrocarbons containing less than about five carbon atoms. Tail gas which represents an overhead product from the Fischer-Tropsch synthesis operation contains the light hydrocarbon fraction plus carbon oxides, hydrogen, and water vapor. Tail gas is generally considered a low value product which may be recycled to the reactor or burned locally as fuel. Although much of the hydrocarbons present in the tail gas are low value hydrocarbons, such as methane, ethane, and propane, the tail gas will also contain significant amounts of butane and propane, as well as some hexane, pentane, and octane. These C4 to C8 hydrocarbons represent commercially valuable products which it would be desirable to recover from the tail gas if an economically attractive means to do so is available. Unfortunately, there are few economically attractive methods for recovering these heavy ends from the tail gas.
Two conventional approaches are available for recovering the heavy ends from the tail gas. Cryogenic cooling may be used to condense and remove the heavy ends from Fischer-Tropsch tail gas. A second approach is to use a circulating lean oil in an adsorption tower to adsorb the hydrocarbons in the tail gas to form rich oil. However, conventional lean oil recovery methods require a regeneration loop where the valuable hydrocarbon products are extracted from the rich oil prior to re-circulating the regenerated lean oil back to the adsorption tower.
The present invention is directed to an integrated process for recovering the heavy ends present in the Fischer-Tropsch tail gas by using the Fischer-Tropsch condensate fraction as a once through lean oil to adsorb the commercially valuable hydrocarbons. The process of the present invention does not require a regeneration loop because the Fischer-Tropsch condensate is not re-circulated. When combined with a stripper, the present process provides an integrated and efficient process for economically recovering the most valuable hydrocarbons from the tail gas while also removing carbon oxides, i.e., carbon dioxide and carbon monoxide, which are also present and are known to cause problems in downstream hydroprocessing operations.
As used in this disclosure the words “comprises” or “comprising” are intended as open-ended transitions meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrases “consists essentially of” or “consisting essentially of” are intended to mean the exclusion of other elements of any essential significance to the composition. The phrases “consisting of” or “consists of” are intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.